matrix: split boost / inject thresholds — kills Shape B cross-pollination

Run #003 surfaced rampant cross-pollination: Q2's "OSHA-30 forklift
Wisconsin" recording (w-4435) became warm top-1 for Q19 (dental
hygienist), Q20 (RN), Q21 (software engineer), and 6 other unrelated
staffing queries. Cause: InjectPlaybookMisses inherited the same
DefaultPlaybookMaxDistance (0.5) as the boost path, but inject is
structurally riskier than boost — boost only re-ranks results that
already retrieved on their own merits, while inject FORCES a result
into top-K, so a loose match cross-pollinates wrong-domain answers.

Empirical motivation from v3:
  Implied playbook hit distances for cross-pollinated cases: 0.20-0.46
  Implied distances for the 6/6 paraphrase recoveries:        0.23-0.30
  Threshold of 0.20 should keep most paraphrases, kill the OOD bleed.

Implementation:
- New DefaultPlaybookMaxInjectDistance = 0.20 in playbook.go.
- New PlaybookMaxInjectDistance field on SearchRequest (override).
- InjectPlaybookMisses signature gains maxInjectDist param; hits whose
  Distance exceeds it are skipped (boost path may still re-rank them).
- TestInjectPlaybookMisses_RespectsInjectThreshold locks the contract
  with one tight + one loose hit, asserting only the tight one injects.
- Existing tests pass explicit threshold (0 = default for tight tests,
  0.5 for the dedupe test which uses 0.30 hits).

Run #004 result on identical queries with the split threshold:

  Verbatim discovery        8 (vs v3's 6 — judge variance, separate)
  Verbatim lift             6 / 8 (75%)
  Paraphrase top-1          6 / 8 (75%)
  Paraphrase any-rank in K  6 / 8

OOD queries Q19/Q20/Q21 ALL show warm top-1 = cold top-1 (no
injection) — cross-pollination eliminated where it was wrong-direction.
Mean Δ top-1 distance dropped from -0.164 (v3, distorted) to -0.071
(v4, comparable to v1's -0.053).

Two paraphrases missed in v4 (Q9, Q15) were ones where qwen2.5
rephrased liberally enough to drift past 0.20 — Q9: "Inventory
specialist..." → "Individual needed for inventory management..." and
Q15: "Engaged warehouse associate..." → "Warehouse associate currently
engaged with a robust history...". The system correctly refusing to
inject when it's not confident is the right product behavior; the
boost path still re-ranks recorded answers when they appear in regular
retrieval.

The Q6 ↔ Q7 cross-pollination ("Forklift-certified loader" ↔
"Hazmat warehouse worker") is legitimate — these are genuinely similar
staffing queries and the judge ranks both directions as plausible.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/matrix/playbook_test.go

@@ -187,7 +187,7 @@ func TestInjectPlaybookMisses_AddsMissingAnswers(t *testing.T) {
 			},
 		},
 	}
-	out, injected := InjectPlaybookMisses(results, hits)
+	out, injected := InjectPlaybookMisses(results, hits, 0)
 	if injected != 1 {
 		t.Fatalf("expected 1 injected, got %d", injected)
 	}
@@ -240,7 +240,7 @@ func TestInjectPlaybookMisses_SkipsAnswersAlreadyPresent(t *testing.T) {
 			},
 		},
 	}
-	out, injected := InjectPlaybookMisses(results, hits)
+	out, injected := InjectPlaybookMisses(results, hits, 0)
 	if injected != 0 {
 		t.Errorf("expected 0 injected (answer already present), got %d", injected)
 	}
@@ -266,7 +266,7 @@ func TestInjectPlaybookMisses_DedupesPerAnswer(t *testing.T) {
 			Entry:      PlaybookEntry{QueryText: "q2", AnswerID: "w-99", AnswerCorpus: "workers", Score: 1.0},
 		},
 	}
-	out, injected := InjectPlaybookMisses(results, hits)
+	out, injected := InjectPlaybookMisses(results, hits, 0.5) // explicit loose threshold so 0.30 hits qualify
 	if injected != 1 {
 		t.Errorf("expected 1 injection (deduped), got %d", injected)
 	}
@@ -280,10 +280,51 @@ func TestInjectPlaybookMisses_DedupesPerAnswer(t *testing.T) {
 	}
 }
 
+// TestInjectPlaybookMisses_RespectsInjectThreshold locks the
+// cross-pollination defense added after run #003: hits whose playbook
+// distance exceeds the inject threshold are skipped, preventing the
+// "OSHA-30 forklift" recording from surfacing as warm top-1 for an
+// unrelated dental-hygienist query just because their text vectors
+// happened to fall within boost-threshold (0.5).
+func TestInjectPlaybookMisses_RespectsInjectThreshold(t *testing.T) {
+	results := []Result{{ID: "w-1", Corpus: "workers", Distance: 0.30}}
+	// Two hits: one within tight inject threshold, one beyond it but
+	// within boost threshold. Only the tight one should inject.
+	hits := []PlaybookHit{
+		{
+			PlaybookID: "tight",
+			Distance:   0.10, // within inject (true paraphrase territory)
+			Entry:      PlaybookEntry{QueryText: "q1", AnswerID: "w-tight", AnswerCorpus: "workers", Score: 1.0},
+		},
+		{
+			PlaybookID: "loose",
+			Distance:   0.40, // boost-eligible but inject-rejected
+			Entry:      PlaybookEntry{QueryText: "q2", AnswerID: "w-loose", AnswerCorpus: "workers", Score: 1.0},
+		},
+	}
+	// Default threshold (0 → DefaultPlaybookMaxInjectDistance = 0.20)
+	out, injected := InjectPlaybookMisses(results, hits, 0)
+	if injected != 1 {
+		t.Errorf("expected 1 injection (only the tight hit qualifies), got %d", injected)
+	}
+	gotTight := false
+	for _, r := range out {
+		if r.ID == "w-tight" {
+			gotTight = true
+		}
+		if r.ID == "w-loose" {
+			t.Errorf("loose hit (distance > inject threshold) was injected anyway")
+		}
+	}
+	if !gotTight {
+		t.Error("tight hit should have been injected")
+	}
+}
+
 // TestInjectPlaybookMisses_EmptyHits is a fast-path no-op check.
 func TestInjectPlaybookMisses_EmptyHits(t *testing.T) {
 	results := []Result{{ID: "w-1", Corpus: "workers", Distance: 0.30}}
-	out, injected := InjectPlaybookMisses(results, nil)
+	out, injected := InjectPlaybookMisses(results, nil, 0)
 	if injected != 0 {
 		t.Errorf("expected 0 injection, got %d", injected)
 	}